The invention described herein was made by an employee of the United States Government, and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties.
The present invention is drawn to determining the attitude and attitude rate of a receiver using signals from the Global Positioning System (GPS) and calculating the Doppler difference between signals using multiple antennae on a receiver.
Global Positioning System devices typically use a phased-locked-loop to track the frequency of GPS satellites. The phase-locked-loop contains a xe2x80x9cPxe2x80x9d (proportional to error) and a xe2x80x9cDxe2x80x9d (proportional to error rate) term, the latter term being used to infer attitude rates. GPS attitude rate determination using the xe2x80x9cDxe2x80x9d term of the phase-locked-loop is noisy. To get any useful information from it, the data has to go through a low-pass filter. This latter operation limits the high-frequency (high rates) that the system can handle.
Alternatively, different devices, such as gyroscopes may be used. A variety of position-determining devices can be used in conjunction with numerical differentiation to yield satellite attitude rates such as star trackers and digital sun sensors. Gyroscopes are very accurate, but are yet another device for the satellite to carry. Also, numerically differentiating attitude-determination is inherently a very noisy operation.
The following patents teach attitude or attitude rate determinations using various methods: U.S. Pat. Nos. 5,101,356; 5,185,610; 5,525,998; 5,268,695; 5,543,804; 5,561,432; 5,757,316; 5,844,521; 5,917,445; 5,933,110; 5,928,309; 5,990,827; 6,101,430; 6,005,514; 6,078,284; 6,133,874; 6,266,582; and xe2x80x9cDevelopment and Flight Demonstration of a GPS Receiver for Spacexe2x80x9d Dissertation by Edgar Glenn Lightsey. All of the above references are hereby incorporated by reference.
The present invention handles both high and low frequency attitude rate determination with high accuracy, for example, a zero crossing vector could be couple with a quartz-based counter, thereby yielding long period estimations to within microseconds). Using Doppler difference to determine attitude rate can replace the need for a gyroscope, thereby allowing cheaper attitude rate determination. Cheaper systems could use GPS-only for positioning, attitude and attitude rate.
The invention is drawn to determining the attitude and attitude rate of a receiver based on signals received by antennae on the receiver. Based on the signal received by the antennae, the Doppler difference between the signals is calculated. With signals received from two signal sources by three antennae pairs, the three-dimensional attitude rate is determined.